XXVI. THE "MIXTUM" OR THE CHEMICAL COMPOUND. AFFINITY BETWEEN THE ELEMENTS.

IN explaining the Physical system we pointed out that, besides the natural tendency or natural appetite which corporeal substances mutually have, as such, we must admit certain particular tendencies whereby some adhere to others and unite with them, to constitute when united diverse other substances which are called mixta, or chemical compounds. These particular tendencies are known by the name of chemical affinities: and we may discuss either their existence or their nature. As to their existence the fact is certain, and so universally admitted, that even those who deny it in theory have to admit it, in fact, at every step, when they mean to discourse, and not talk nonsense, about the changes of corporeal substances. We shall not waste words about that, but say something about their natures.

And here it is well to remember what was said before about mechanical motion, which is forced, because extrinsically produced, and therefore has no natural direction or term fixed at which the motion would cease. No one will deny the fact that every corporeal substance may be subjected to mechanical impulses, and then moved ab extrinseco: but to suppose that chemical affinities originate from them would be utterly unreasonable and not worth a serious argument. God willed that in corporeal nature there should be a various and continual succession, in which, as St. Augustine says, "the morning of things always comes after the evening." Therefore He gave to the elementary substances that active principle by which they are constituted in their specific being, and from which those particular tendencies follow that go by the name of affinities. Moreover, as if to assure us of the fact that even beings without knowledge have these particular tendencies, He willed that in beings there should be a gradual descent from the most perfect animal down to inorganic substance, not only without knowledge but also without life. Common sense tells us that the movement of a horse or of a lion to a certain spot is by an inner principle of nature but when, in the order of sensitive beings, we come to the lowest species, such as zoophytes, many people find it hard to see how certain motions can be accounted for on the same principle. Nevertheless analogy alone ought to persuade us of the fact, and make us listen to the suggestions of reason, instead of being carried away by the allurements of imagination. But if this applies to zoophytes, it also applies to inorganic substances tending towards a certain determinate end so that sound reasoning rejects the mere external principles of mechanical motion as quite improbable. If it be absurd, as in fact it is, to say that a stone pitched at a man is attracted by the man hit, equally absurd is it to assert, as some chemists do, that the elementary atoms are moved and directed to unite by external impulses, and yet have a mutual affinity.

We must remark that chemical affinities take place between those substances only which, when afterwards combined, give, as the result of the combination, another substance, whose nature differs from theirs. And since, as we have said, the components also must therefore be of diverse natures, chemical affinities therefore cannot take place otherwise than between elementary substances differing from each other.

This affinity may be subject to accidental modifications: for if plants, brutes, men and angels are subject to accidental changes while retaining their own nature, how can we except elementary atoms, placed in the lowest grade of being? The Physical system says that we cannot. Reason and experience give the same answer. For in an elementary atom there is extension and the principle of activity or force: and the former can undergo changes in the more or less, while the latter can be altered by contrary force. Thus we can see clearly that elements operate under some circumstances, and under others do not, or that, if they do, their operation is weak, though not specifically different. No wonder then if the different states, if the different modifications to which the atoms are subject, either impede or promote the appearance of chemical affinities: for this is quite natural, as we may judge by the analogy between the proper tendencies of living beings and the affinities of things without life. Hence the common saying of chemists that affinity varies very much, according to the various dispositions in which bodies are found. In fact the elements that have affinity to each other cannot combine otherwise than by mutual operation, to which they either are, or are not disposed in themselves. If they are so, proximity and contact will suffice to set going their reciprocal action. If not, they must be disposed from without. Hence the necessity of an extrinsic agent, to operate on them and alter them, as heat and electricity are known to do. Some, it is true, have asserted that the mere presence of some determinate substance is sufficient to make the elements operate: but if this is to be understood strictly, so as not to imply true action, it is trifling and false. What is it present for, if it does nothing? To introduce that in chemical combinations for the purpose of awakening affinity, is out of the question, and multiplies beings without necessity, against the well known axiom, Entia non sunt multiplicanda sine necessitate.

And let this be sufficient about affinity, according to the principles of the Physical system: for we have shown that system to be in conformity with science and explanatory of the facts that science undertakes to investigate therein.